cgroups/systemd: fix set CPU quota if period is unset
cgroups/systemd: fix set CPU quota if period is unset

systemd drivers ignore --cpu-quota during update if the CPU
period was not set earlier.

Fixed by adding the default for the period.

The test will be added by the following commit.

Signed-off-by: Kir Kolyshkin <3a017b8ddb3f9cf3e4a59978b004111bdeb97f08@gmail.com>

mikebrow/runc,opencontainers/runc,kolyshkin/runc,kolyshkin/runc,rhatdan/runc,rhatdan/runc,mikebrow/runc,estesp/runc,dqminh/runc,xiaochenshen/runc,estesp/runc,dqminh/runc,opencontainers/runc,opencontainers/runc,xiaochenshen/runc,mikebrow/runc,xiaochenshen/runc,dqminh/runc,kolyshkin/runc,rhatdan/runc,rhatdan/runc,estesp/runc,dqminh/runc,kolyshkin/runc,opencontainers/runc